1. Field of the Invention
The present invention relates to a thermal field emission cathode which is employed in an electron microscope, a critical dimension machine, an electron beam lithograph machine, an electron beam tester and the like.
2. Discussion of Background
In recent times, a thermal field emission cathode has been developed which utilizes a needle-like electrode of single crystal tungsten to provide an electron beam having a higher brightness. In a so-called ZrO/W thermal field emission cathode wherein a tungsten tip (hereinafter W tip) having an axis direction of &lt;100&gt; is provided with a coating layer composed of zirconium and oxygen, and the work function of a (100) surface is selectively lowered from 4.5 eV to 2.8 eV by a ZrO coating layer. Accordingly, compared with a conventional thermionic cathode, the thermal field emission cathode is provided with a high brightness and a long service life, and provided with a characteristic wherein the thermal field emission cathode is more stable and easier to use than a field emission cathode.
FIG. 2 shows a sectional diagram of a thermal field emission cathode. A reference numeral 1 designates a W tip, 2, a suppressor electrode for supplying a voltage to form an electric field which restrains the emission of thermions, 3, a tungsten wire which is a heater for heating the W tip and 4, an insulator. A numeral 5 designates a metal support. Further, FIG. 1 is a diagram magnifying a portion of the W tip 1 and the tungsten wire 3 of FIG. 2. A portion of the W tip is provided with a supply source 7 for supplying zirconium and oxygen. The surface of the W tip is covered with a ZrO coating layer, although not shown.
The W tip 1 of the ZrO/W thermal field emission cathode is heated by the tungsten wire 3 by flowing current in the tungsten wire 3, and is utilized under a temperature of approximately 1,800K. Therefore, the W tip 1 is consumed by evaporation. However, zirconium and oxygen are continuously supplied to the surface of the W tip 1 from the supply source 7 for supplying zirconium and oxygen through a surface diffusion thereby continuously forming the ZrO coating layer. When zirconium or oxygen in the supply source 7 for supplying zirconium and oxygen is exhausted, the formation of the ZrO coating layer is finished, the work function is increased, the function as the thermal field emission cathode is lost and its service life expires.
A conventional method for forming a ZrO coating layer to provide a low work function is shown as follows (U.S. Pat. No. 4,324,999).
First step: Powders of zirconium hydride (ZrH.sub.2) as a precursor of a substance including zirconium is added with an organic solvent to thereby form a slurry, and a storage of zirconium hydride is formed by adhering the slurry to the W tip 1 having a direction of &lt;100&gt;.
Second step: Heating of the W tip 1 is performed in a high vacuum, zirconium hydride is decomposed into zirconium and hydrogen, and zirconium is diffused into the W tip 1.
Third step: The W tip 1 is heated in an atmosphere of oxygen of approximately 10.sup.-6 Torr, and a ZrO coating layer is formed on the W tip 1. At the same time, the total quantity of zirconium or a portion thereof is transformed into zirconium oxide and a supply source of zirconium and oxygen is formed (hereinafter, this step is called oxidation treatment).
However, there are several problems in making the ZrO/W thermal field emission cathode by the conventional method.
The first problem in making the ZrO/W thermal field emission cathode by the conventional method is that a variation of the service life is large, the occurrence of failure as in melting and cutting-off of the tungsten wire 3 and the destruction of the W tip 1 by discharge, is high and an unstable behavior of the electron beam is often observed. For instance, when the supply source 7 of zirconium and oxygen is disposed in the vicinity of the W tip 1 on the side of the heater, a temperature thereof is elevated, evaporating rates of zirconium and oxygen are accelerated and the service life is shortened. Especially, in case wherein the oxidation treatment is insufficient and an amount of unoxidized zirconium is large, this effect is significant, since the vapor pressure of zirconium is about fifty times as much as the vapor pressure of oxidized zirconium at 1,800K. Further, when the supply source 7 of zirconium and oxygen is disposed at the junction of the W tip 1 and the tungsten wire 3, or on the tungsten wire, the heating characteristic changes, the heating current is more necessary and the tungsten wire 3 may be molten and cut off in an extreme case.
On the other hand, when the supply source 7 of zirconium and oxygen is disposed in the vicinity of the distal end of the W tip 1, the temperature is low, the evaporation rates are slow, which is preferable in view of the service life. However, this arrangement disturbs an electric field distribution in the vicinity of the distal end of the W tip 1 and an opening of the suppressor electrode 2, or the electron beam becomes unstable, which considerably lowers the function of the electron emission cathode. However, no investigation has been performed with respect thereto.
Further, the second problem is that the supply source of zirconium and oxygen is peeled off from the W tip in the step of making and the product can not be provided. Or, even if the product is provided, the peeling-off is caused in use thereof, which causes an inconvenience wherein the function of the thermal field emission cathode is lost, and the like.
The third problem is that a desired electron beam characteristic is not provided.
FIG. 3 is a schematic diagram showing an electric circuit whereby a thermal field emission cathode is employed. The thermal field emission cathode generates a spatially diverging electron beam. However, in an end use such as an electron microscope, an electron beam I.sub.p 9 at the axis center portion is mainly employed. To provide a quantity of electron beam at this portion, the temperature and the extraction voltage Vex 10 of the thermal field emission cathode are controlled. However, an increase in the extraction voltage increases not only the electron beam I.sub.p 9 at the axis center portion but also the electron beam quantity at the surrounding portion. When the total electron beam quantity is larger than necessary, a large capacity of power supply is necessary for operating the thermal field emission cathode, which is economically disadvantageous. With respect to an already designed and manufactured device employing the thermal field emission cathode, a thermal field emission cathode having a characteristic wherein the electron beam quantity at the surrounding portion is large compared with the electron beam quantity I.sub.p 9 at the axis center portion, can not be employed. Further, when the total electron beam quantity I.sub.t 12 is large, a gas discharge quantity from the electrode material is increased by the electron bombardment effect. Therefore, the electron beam becomes unstable over time and spatially. The electron beam quantity at the surrounding portion or, accordingly, the total electron beam quantity is preferable to be as small as possible, so far as a desired electron beam quantity at the axis center portion can be provided.